This invention relates to a method for forming a slurry from a difficult to wet, finely divided powder and an aqueous liquid and, more particularly, to the formation of a slurry of partially calcined alumina dust and Bayer process spent liquor.
In the production of alumina via the Bayer process involving calcining of the alumina trihydrate precipitated in the Bayer process, a substantial amount of alumina dust is produced. The recovery and/or elimination of this alumina dust is of major environmental and economic interest. Alumina dust in the production of alumina via the Bayer process is usually recovered by mechanical and/or electrostatic devices, such as filters, cyclone dust separators and electrostatic precipitators. The resultant recovered dust is usually partially calcined and has a very fine particle size, particularly the alumina dust recovered from the electrostatic precipitators which might have a particle size such that at least 90% by weight of the partially calcined alumina dust is smaller than about 44 microns (325 mesh), more or less.
The disposal of this dust has always been a difficult problem. Since it is very finely divided and undercalcined, it cannot be included in the normal production of calcining kilns for sale and use in smelters. It is possible to add the dust to pregnant Bayer liquors to act as fine seed and be grown to a larger useful size. It may also be added to spent liquor and processed through a classification circuit to be separated as fine seed and returned to the beginning of the precipitation circuit for further growth.
The following are examples of prior systems:
Gnyra, U.S. Pat. No. 4,051,222, describes a process to coarsen alumina dust produced in calciners for reuse in the Bayer process. This is accomplished by mixing the dust with Bayer process pregnant liquor and calcium carbonate in an aqueous medium. The produced slurry containing up to about 50 grams calcium carbonate per liter of pregnant liquor is used as seed for alumina trihydrate precipitation from pregnant Bayer process liquor.
Anjier et al, U.S. Pat. No. 4,568,527, describes a process in which alumina dust is employed as seed for the precipitation of alumina trihydrate from pregnant Bayer process liquors. For this purpose, the dust is slurried with pregnant liquor to give a concentration of solids equivalent to 2-10m.sup.2 /L which at a surface area of 3.9 m.sup.2 /g (Col. 5, lines 36-38), is 0.5-2.5 g/L. This slurry is fed to the beginning of the Bayer 10 process precipitation circuit.
The prior systems require that the dust be slurried in the liquor. Due to the very fine particle size of the dust and its resistance to wetting, attempts to add the dust to liquid under mechanical mixing with a stirrer have created intolerable dusting problems. Moreover the product has tended to form into large lumps, only the outside of which have been fully wetted, resulting in an impermeable outer layer surrounding a core of dust.
It is the object of the present invention to provide a method to readily form a slurry of any finely divided powder that is difficult to wet with a liquid and, particularly, to form a slurry of partially calcined alumina dust and Bayer process spent liquor.